The invention relates to a process for identifying an embossed image of a coin in an automatic coin tester.
DE 330 55 09 A1 has made known an optical coin validation device. Here, a coin is illuminated from a light source at an angle from the surface perpendicular and the reflected light is recorded by two radiation receivers. The images each picked up by the radiation receivers are compared to each other to determine the degree of coin gloss. To this end, the values of the signals are divided by each other. The quotient obtained for the coin indicates the degree of gloss which is used to differ between the genuine and counterfeit natures of the coin.
U.S. Pat. No. 5,839,563 has made known an optical coin tester which identifies a coin on the basis of a first pick-up of the light reflected from the embossed image and a second pick-up of the light reflected from the border of the coin. Also, a proposal is made to record images of the opposed sides of the coin and to employ them for coin identification. The coin to be identified lies on a glass pane and is lit up by ring-shaped lighting sources from below. The embossed image which is picked up is transformed into polar coordinates and the angle is determined at a maximum intensity for a radius value. The angle is compared to the recorded reference, which determines the angle of rotation. It is not the topological profile of the coin which is evaluated here, but it is an intensity profile instead.
It is object of the invention to provide a process for identifying an embossed image of a coin in an automatic coin tester which permits to identify the coin by simple means in a reliable manner.
According to the invention, the object is attained by the process steps of claim 1. Advantageous aspects constitute the subject of the sub-claims.
In the inventive process, the coin to be identified is moved to an image receiver and a light source. The image receiver picks up at least one image of the embossed image of the coin. A measuring device compares the image to a first reference pattern to the effect whether the first reference pattern is contained in the image which was picked up. In an aspect, a genuine coin or counterfeit coin signal is produced already as a result of a comparison to a first reference pattern.
If the validation device finds the first reference pattern in the image which was picked up the validation device preferably determines whether a second reference pattern is contained in a predetermined area the location of which is determined relative to the position of the first reference pattern. In this step, the region of search for the second reference pattern is restricted on the basis of the position of the first reference pattern. This causes the search for the second reference pattern to focus on relevant areas and reduces the procedure of search. Because of a coincidence of the image with the reference pattern(s), the validation device produces a genuine coin or counterfeit coin signal for the coin to be validated with further reference parameters being adapted to be taken into account. In the inventive process, it is preferred that two reference patterns be compared to the image which was taken up. A determination is made for the two reference patterns as to whether they are contained in the image which was taken up. Further, a determination is made for the two reference patterns whether they are contained in the image which was taken up, in the predetermined position relative to each other. This restricts the region of search for the second reference pattern, on one hand, but takes into account an extra information on the relative position of the reference patterns with respect to each other. The result is that a particularly reliable outcome may be obtained approximately rapidly.
It is appropriate that the first and second reference patterns concerned be partial images forming the embossed image. The inventive process allows to employ more reference parameters of the coin and its embossed image to determine the genuine coin or counterfeit coin signals.
In a preferred aspect, the light source illuminates the embossed image from a plurality of directions and the image receiver records a separate image of the embossed image for each direction of lighting. If one-colour lighting is used the embossed image is illuminated from different directions in a successive time sequence and one image each is picked up. If multi-colour lighting is used it is possible to pick the images up simultaneously with each image receiver then being responsive to one colour only and providing one pick-up by one lighting. In a very specific aspect which is preferred, a differential image is produced which, as the image picked up, is compared to the reference pattern. The processing of images using illumination from different directions and the production of a differential image is referred to as a Selective Stereo Gradient Process (SSG process). The SSG process, because of an illumination from different directions, cannot be deluded by the photo of an embossed image because the pick-up of a photo does not change, when illuminated from different directions and, hence, the differential image will neutralize itself.
In an appropriate further aspect, the image which is picked up is binarized, i.e. converted into a image which merely has two signal types for black and white. A uniform threshold value, e.g. the grey-scale mean value of the image picked up, may be employed to binarize the image. Likewise, it is possible to accomplish binarization with local threshold values, e.g. by using a maximum-enthropy method.
It is preferred to determine the threshold value according to the maximum enthropy method where the threshold value 0xe2x89xa6txe2x89xa6255 is chosen such as to make the xe2x80x9ctotalxe2x80x9d enthropy a maximum:   H  =            -                        ∑                      i            =            0                    t                ⁢                  xe2x80x83                ⁢                              p            i                    ⁢                      log            e                    ⁢                      p            i                                -                  ∑                  j          =                      i            +            1                          255            ⁢              xe2x80x83            ⁢                        p          j                ⁢                  log          e                ⁢                  p          j                    
where pj is the fraction of the pixels having the grey-scale value i. The first term may be construed to be as a bright foreground and the second term as a dark background here.
In a preferred aspect of the process, the image receiver picks up an overall image of the embossed image from which the validation device determines the diameter of the coin to be identified. The diameter may be employed by the validation device as a further magnitude to discriminate a counterfeit coin. In a further aspect of the process, the validation device also determines the midpoint of the embossed image in the overall image and transforms the overall image into polar coordinates wherein a first coordinate indicates the distance from the midpoint and the second coordinate indicates an angle of the radius beam from an orientation determined for the whole image, for any point in the embossed image.
Such transformation into polar coordinates, apart from ensuring an identification process which saves computation time and is rotation and translation invariant, also has the advantage that it is easy to restrict and locate regions of search for the reference patterns. Thus, for instance, a region of search for the first reference pattern may be restricted already by predetermining a radius interval for a search for this region. In such a case, an angle interval would not be preset.
For a precise determination of the midpoint during transformation, three or more locations are determined on the coin border from which the midpoint is determined for the image which was picked up.
In a preferred further aspect of the process, the reference patterns to be searched for are preferably selected randomly from a multiplicity of reference patterns. Thus, at the beginning of the process, it is not sure which features to examine on the coin to be tested. Likewise, the inventive process allows to vary the parameters for an identification of a counterfeit coin following a previously generated counterfeit coin signal such that a deviation from the reference patterns will lead earlier to the production of a counterfeit coin signal. A coin is identified as a counterfeit coin earlier if the requirements for identifying a counterfeit coin signal are decreased. This step makes it possible that if counterfeit money is inserted in the automatic coin tester the requirements for producing a genuine coin signal are increased. Accordingly, a provision can be made to decrease the requirements again after a multiplicity of coins which were discriminated as being genuine.
In an advantageous aspect which is specifically recommended for embossed images of a very fine structure more than two reference patterns are searched for in the image which was picked up with the region of search a further reference pattern resulting in dependence on the regions of search for the preceding reference patterns.